1. Technical Field
The present invention relates to an apparatus for helping to protect a vehicle occupant in the event of a collision. In particular, the present invention relates to an energy management device for seat belt webbing used in a vehicle seat belt system.
2. Description of the Prior Art
Seat belt systems are commonly used to restrain vehicle occupants. A typical seat belt system for restraining a vehicle occupant includes a length of belt webbing extensible about the occupant and having opposite ends anchored to the vehicle. In the event of sudden deceleration such as occurs in a vehicle collision, the occupant tends to move forward relative to the vehicle and is restrained by the belt webbing.
The vehicle occupant can experience a sudden load if the occupant's forward movement is restrained by an inextensible length of belt webbing. To alleviate this sudden load, it is known to provide an energy management device in the seat belt system. When the occupant engages the belt webbing with sufficient force, the energy management device allows the belt webbing to extend slightly. The occupant's movement is restrained at a reduced rate over an increased period of time. This can reduce the maximum load experienced by the vehicle occupant.
One known type of energy management device is described in U.S. Pat. No. Re. 29,147 and includes a length of belt webbing wound on a spool. The spool is mounted on a frame that also includes a plastically deformable torsion bar. When the force on the belt webbing exceeds a predetermined amount, the torsion bar twists, and the spool rotates, enabling the belt webbing to be unwound from the spool.